The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (SI) engines introduce an air/fuel mixture into each cylinder which is compressed in a compression stroke and ignited by a spark plug. Known compression ignition engines inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke which ignites upon injection. Combustion for both gasoline engines and diesel engines involves premixed or diffusion flames controlled by fluid mechanics.
SI engines can operate in a variety of different combustion modes, including a homogeneous SI combustion mode and a stratified-charge SI combustion mode. SI engines can be configured to operate in a homogeneous-charge compression-ignition combustion mode, also referred to as controlled auto-ignition combustion, under predetermined speed/load operating conditions. The controlled auto-ignition combustion comprises a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the controlled auto-ignition combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. Controlled auto-ignition combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of an air/fuel stoichiometric point, with relatively low peak combustion temperatures, resulting in low NOx emissions. The homogeneous air/fuel mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
Controlled auto-ignition combustion depends strongly on factors such as cylinder charge composition, temperature, and pressure at intake valve closing. Hence, the control inputs to the engine must be carefully coordinated to ensure auto-ignition combustion. Controlled auto-ignition combustion strategies may include using an exhaust recompression valve strategy. The exhaust recompression valve strategy includes controlling a cylinder charge temperature by trapping hot residual gas from a previous engine cycle by adjusting valve close timing. In the exhaust recompression strategy, the exhaust valve closes before top-dead-center (TDC) and the intake valve opens after TDC creating a negative valve overlap (NVO) period in which both the exhaust and intake valves are closed, thereby trapping the exhaust gas. The opening timings of the intake and exhaust valves are preferably symmetrical relative to TDC intake. Both a cylinder charge composition and temperature are strongly affected by the exhaust valve closing timing. In particular, more hot residual gas from a previous cycle can be retained with earlier closing of the exhaust valve leaving less room for incoming fresh air mass, thereby increasing cylinder charge temperature and decreasing cylinder oxygen concentration. In the exhaust recompression strategy, the exhaust valve closing timing and the intake valve opening timing are measured by the NVO period.
In addition to a valve control strategy, there must be a suitable fuel injection strategy for combustion. At low fueling rates (e.g., <7 mg/cycle at 1000 rpm in an exemplary 0.55 L combustion chamber volume) the cylinder charge may not be hot enough for controlled auto-ignited combustion even with maximum allowable NVO, leading to partial-burn or misfire.
It is known to increase the cylinder charge temperature by pre-injecting a small amount of fuel when a piston approaches TDC intake during the recompression portion of NVO. A portion of the pre-injected fuel reforms due to high pressure and temperature during the recompression portion, and releases heat energy, increasing the cylinder charge temperature enough for complete controlled auto-ignition combustion of the combustion charge resulting from the subsequent main fuel injection. The amount of such fuel reforming is based upon the pre-injection mass and timing, typically with fuel reforming increasing with earlier pre-injection timing and greater pre-injection fuel mass.
However, excessive fuel reforming to increase operability range of controlled auto-ignition can increase combustion instability and thereby decreases fuel efficiency. Therefore, it would be advantageous to extend operating ranges for controlled auto-ignition combustion without incurring combustion instability, and therefore increasing fuel efficiency.